What is the CommonJS specification? Understand the CommonJS specification

This article brings you what is the CommonJS specification? Understanding CommonJS specifications has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

Write in front:
A file is a module.
In addition, the sample code in this article needs to run normally in the node.js environment, otherwise an error will occur. In fact, ES6 has already appeared in the module specification. If you use the ES6 module specification, you do not need a node.js environment. Therefore, the commonJS specification needs to be distinguished from the ES6 module specification.

1. Why was the CommonJS specification developed?

On the backend, the JavaScript specification is far behind and has many flaws, which makes it difficult to develop large-scale applications using JavaScript. For example:

No module system
Less standard library
No standard interface
Lack of package management system
List content

CommonJS specification is proposed mainly for To make up for the lack of standards in JavaScript, it has reached the stage of developing large-scale applications like Python, Ruby and Java, instead of staying at the stage of developing small browser-side scripts.

2. CommonJS module specification
The CommonJS module specification is mainly divided into three parts: module reference, module definition, and module identification.

1. Module reference

If you use the following statement in a.js file

var math=require（'math');

means using the require() method, introduce the math module, and assign it to the variable math . In fact, the named variable name and the imported module name do not have to be the same, just like this:

var Math=require('math');

The significance of assignment is that only Math will be recognized in a.js, because this is a variable that has been defined and cannot Recognize math because math is not defined.

With path and without path

In the above example, the require parameter is only the string of the module name, without a path, and the reference is a The math module in the node_modules directory in the current directory where .js is located. If there is no node_modules directory in the current directory or the math module is not installed in the node_modules directory, an error will be reported.
　
If the module to be imported is in another path, you need to use a relative path or an absolute path, for example:

var add=require('./add.js')

In the above example, the add.js file in the current directory is introduced and assigned a value Give add variable.

2. Module definition

module object: In each module, the module object represents the module itself.
Export attribute: an attribute of the module object, which provides an interface to the outside world.

Still using the previous example, assume that the code in add.js is as follows:

function add(num1,num2){
    alert(num1+num2);
}

Although the a.js file introduces the add.js file, the former still cannot use the add in the latter. Function, code like add(3,5) in a.js file will report an error, indicating that add is not a function.
　For the functions in add.js to be used by other modules, they need to expose an external interface. The export attribute is used to complete this work. Change the code in add.js to the following:

exports.add=function (num1,num2){
    alert(num1+num2);
}

The a.js file can call the methods in add.js normally. For example, calls like add.add(3,5) can be executed normally. The previous add means the module represented by the add variable in this file, and the latter add is the add method that introduces the module.

3. Module identification

The module identification refers to the parameters passed to the require method, which must be a string named in camel case, or in the form of ., .. , a relative path at the beginning, or an absolute path.

3. Benefits of CommonJS module specification
CommonJS module specification solves the problem of variable pollution very well. Each module has an independent space and does not interfere with each other. Namespace and other solutions are used with it. It pales in comparison.

CommonJS standard definition module is very simple, and the interface is very concise.

CommonJS module specification supports import and export functions, so that each module can be smoothly connected and dependencies between each other can be realized.

The above is the detailed content of What is the CommonJS specification? Understand the CommonJS specification. For more information, please follow other related articles on the PHP Chinese website!